A rotary fluid machine disclosed in Japanese Patent Application Laid-open No. 2000-320543 is equipped with a vane piston unit in which a vane and a piston are combined; the piston, which is slidably fitted in a cylinder provided radially in a rotor, interconverts the pressure energy of a gas-phase working medium and the rotational energy of the rotor via a power conversion device comprising an annular channel and a roller, and the vane, which is radially and slidably supported in the rotor, interconverts the pressure energy of the gas-phase working medium and the rotational energy of the rotor. The rollers of the power conversion device are formed from grease-packed ball bearings provided on support shafts projectingly provided on the left and right sides of the vane; an inner race of the bearing is fixed to the support shaft, and an outer race thereof is rollably engaged with the annular channel.
Since the vane piston unit of such a rotary fluid machine operates in a harsh environment in which it is exposed to a high temperature, high pressure gas-phase working medium, there are the problems of not only the durability of the rollers themselves being degraded due to leakage of the grease packed in the rollers, but also the working medium being contaminated by the grease that has leaked from the rollers. Since oil that has contaminated the working medium carbonizes at a relatively low temperature (e.g., 180° C.), the carbonized material thus formed builds up in an evaporator or a condenser of a Rankine cycle system and becomes a main factor in degrading the performance, which is a problem. Moreover, since the oil that has contaminated the working medium gasifies at a low temperature, an extensive filter system is required in order to remove the gasified oil from the working medium, and this is also a problem.